The present invention is directed to ink compositions and imaging and printing processes thereof. More specifically, in one embodiment the present invention is directed to fluorescent ink jet ink compositions useful in the printing of concealed images for security or encoding applications, and wherein the inks contain dyes based on the porphyrin chromophore, or analogous components. These dyes usually possess an extremely strong absorption band at 400 to 500 nanometers, the Soret Band, in addition to much weaker bands in the 500 to 800 nanometers range. These dyes also exhibit fluorescence in the 600 to 800 nanometers spectral range, an area which is distinct from the emission window characteristic of the optical brightners used in commercial papers. In one embodiment the inks of the present invention are comprised of a porphyrin chromophore dye like tetrapyridinium porphyrin tetra-acetate and an aqueous liquid vehicle. The inks can be formulated after mixing the aqueous component and the dye by the addition, for example, of a cosolvent comprised of water and a glycol, like diethyleneglycol, thereby improving latency, which is the maximum time period, for example less than one hour, and from about 1 to about 10 minutes, over which an uncapped ink jet printhead can remain idle before noticeable deterioration of its jetting performances, and this addition can improve ink drying time, that is the time needed for an ink jet print to dry to an extent such that it will not smear or offset upon handling or when placed in contact with another sheet of paper, which drying time can, for example, be less than one minute, or more specifically from about 10 to about 30 seconds. Also, the addition of glycol permits the adjustments of the ink viscosity from about 1.1 to about 4 centipoises, and preferably from about 1.1 to about 3.0 centipoises, and can permit adjustment of the ink surface tension. Viscosity and surface tensions are major contributing factors in the production of excellent quality prints on plain papers, that is prints with acceptable edge acuity, that is the sharpness of the image between the printed and nonprinted areas, minimal ink feathering on paper, and characterized, for example, by a desirable uniformity of solid area ink coverage. The inks of the present invention can be selected for a number of known ink jet printing methods and apparatus, including thermal ink jet or bubble jet processes as described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of which are totally incorporated herein by reference.
Ink jet printing systems can generally be classified by two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, they are much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The second type of drop-on-demand system is known as thermal ink jet or bubble jet. With this type, there are apparently generated high velocity droplets and there is allowed very close spacing of the nozzles. The major components of this type of drop-on-demand system are an ink-filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction toward a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known as indicated herein, and are described, for example, in U.S. Pat. Nos. 4,601,777; 4,251,824; 4,410,899; 4,412,224; and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Known ink jet inks generally comprise a water soluble dye which is soluble in an ink vehicle such as water or a mixture comprising water and a water soluble or water miscible organic solvent. Inks comprising soluble dyes may exhibit many problems, such as poor waterfastness, poor lightfastness, clogging of the jetting channels as a result of solvent evaporation and changes in the solubility of the dye, dye crystallization, ink bleeding when prints are formed on plain papers, poor thermal stability, chemical instability, ease of oxidation, and low drop velocity. In addition, many of the dyes contained in inks may be potentially toxic or mutagenic. Also, with the inks of the present invention there are selected dyes that enable the verification of documents printed on fluorescent paper with such inks, primarily because of the emission spectra of the dyes as indicated herein.
The following United States patents are also mentioned: U.S. Pat. No. 4,705,567 relating, for example, to heterophase ink compositions comprised of water and a dye covalently attached to a polyethylene glycol, or polyethylene imine component, which component is complexed with a heteropolyanion; U.S. Pat. No. 4,623,689 which discloses, for example, an ink for ink jet recording wherein the ink contains a certain aqueous colored polymer, see the Abstract for example; and as collateral interest U.S. Pat. Nos. 4,664,708; 4,680,332 and 4,791,165. The disclosures of the aforementioned patents, and all other patents mentioned herein are totally incorporated herein by reference.
Copending application U.S. Ser. No. 544,564 (now abandoned), the disclosure of which is totally incorporated herein by reference, relates, for example, to ink compositions which comprise an aqueous liquid vehicle and colored particles of an average diameter of 100 nanometers or less which comprise micelles of block copolymers of the formula ABA, wherein A represents a hydrophilic segment and B represents a hydrophobic segment, and wherein dye molecules are covalently attached to the micelles. In a specific embodiment of the copending application, the colored particles comprise micelles of block copolymers of the formula ABA having silica precipitated therein and dye molecules covalently attached to the micelles.
U.S. Pat. No. 5,225,900, entitled "Method of Storing Information Within a Reprographic System", with the named inventor Joseph D. Wright, the disclosure of which is totally incorporated herein by reference, discloses apparatuses and processes for controlling a reproduction system by scanning an image to detect at least one taggant in at least one marking material forming the image and issuing instructions to the reproduction system; the instructions cause the reproduction system to take an action selected from the group consisting of (a) prohibiting reproduction of those portions of the image formed by a marking material containing at least one predetermined detected taggant and reproducing all other portions of the image; (b) prohibiting reproduction of any part of the image upon detection of at least one predetermined taggant; (c) reproducing only those portions of the image formed by a marking material containing at least one predetermined taggant; (d) reproducing portions of the image formed by a marking material containing at least one predetermined taggant in a different manner from that in which the system reproduces portions of the image formed by a marking material not containing at least one predetermined taggant; and (e) identifying a source of the image on the basis of detection of at least one predetermined taggant.
Although known ink compositions are suitable for their intended purposes, a need remains for ink compositions that are invisible to the human eye under normal viewing conditions but readable by a sensor, such as an infrared detector or a fluorescence detector, or by the human eye under special viewing conditions such as illumination of the developed image with ultraviolet light. Further, there is a need for ink compositions that can provide a means for placing coded information on a document. Further, there is a need for ink compositions that are nontoxic and nonmutagenic. There is also a need for ink compositions that can be prepared by simple and economical processes. Further, there is a need for ink compositions suitable for printing on plain papers, coated or treated papers, and transparency materials. In addition, there is a need for ink compositions that when printed on substrates exhibit low feathering, and excellent rub-resistance. A need also remains for ink compositions with acceptable thermal and oxidative stability.